1. Field of the Invention
The present invention relates to a wire-wound apparatus, such as a transformer or a reactor, which has a winding wound around a magnetic core and is used in an insulating cooling medium, and more particularly to a shape of an insulation easing member (hereinafter called as the corona ring) of such wire-wound apparatus.
The present invention can be applied to a saturable reactor, a step-up transformer or the like of a magnetic pulse compression circuit for generating a high-voltage pulse used for a discharge pumped laser, an apparatus for decomposing a compound by an electric discharge or sterilizing, or the like.
2. Description of the Related Art
A discharge pumped laser, a device for decomposing a compound such as dioxin by performing a pulse corona discharge, a pasteurizer for sterilizing food or the like by an electric discharge, or the like has discharging electrodes disposed within a discharge cell (chamber) and causes a discharge by applying a high-voltage pulse to the discharging electrodes. As a circuit for generating such a high voltage, a high-voltage pulse generating circuit using a magnetic compression circuit or the magnetic compression circuit and a step-up transformer circuit is generally known.
For example, the discharge pumped laser such as an excimer laser, a fluorine laser or the like oscillates pulse laser by repeatedly discharging between the discharging electrodes in a short time.
It is necessary to supply the discharging electrodes with a high voltage in a short time, and a high-voltage pulse generating circuit is disposed therefor. As a high-voltage pulse generating circuit for the discharge pumped laser, the aforesaid magnetic pulse compression circuit is generally used.
FIGS. 13(a) and 13(b) show configurations of general high-voltage pulse generating circuits disposed in a discharge pumped laser or the like. The configuration of FIG. 13(a) is an example including a two-stage magnetic pulse compression circuit using magnetic switches SR2, SR3 consisting of a saturable reactor, and the two-stage magnetic pulse compression circuit is indicated in a square of a dotted line in the drawing. FIG. 13(b) shows an example including a step-up transformer in addition to the above magnetic compression circuit, and a step-up transformer Tr is disposed instead of the reactor L1 of FIG. 13(a).
An operation of the high-voltage pulse generating circuit shown in FIG. 13(a) will be described. The operation of FIG. 13(b) is the same as that shown in FIG. 13(a) except that the voltage is increased by the step-up transformer Tr. Therefore, its description is omitted.    (1) An electric charge is charged from a high-voltage power supply (charger) to a capacitor C0 via the inductance L1.    (2) A switch SW is a semiconductor switch and, for example, an IGBT is used. When the semiconductor switch SW is closed to turn on, a current flows to a loop of the main capacitor C0, a magnetic switch SR1, the solid-state switch SW and a capacitor C1, and the electric charge of the capacitor C0 transfers to the capacitor C1.    (3) At the time, because a high voltage of 20 to 30 kV is applied to the charged capacitor C0, the same voltage is also applied to the semiconductor switch SW when the switch is turned on. A module of the semiconductor switch SW generally has a rated voltage of several kV, so that a plurality of modules of the semiconductor switch SW are connected in series to configure a switch circuit.    (4) When an integral value of a voltage of the capacitor C1 with time reaches a limit value which is determined according to the properties of a magnetic switch SR2, the magnetic switch SR2 is saturated, the current flows to a loop of the capacitor C1, a capacitor C2 and the magnetic switch SR2, and the electric charge of the capacitor C1 transfers to the capacitor C2. At this time, a pulse width of the current is compressed.    A compression ratio of the pulse width depends on the number of turns of a wire wound around the core of the magnetic switch SR2. Such a circuit is called a magnetic pulse compression circuit.    (5) Then, when an integral value of voltage V2 of the capacitor C2 with time reaches a limit value which is determined according to the properties of a magnetic switch SR3, the magnetic switch SR3 is saturated, the current flows to a loop of the capacitor C2, a peaking capacitor CP and the magnetic switch SR3, the electric charge of the capacitor C2 transfers to the peaking capacitor CP, and the peaking capacitor CP is charged. At this time, a pulse width of the current is compressed. A compression ratio of the pulse width depends on the number of turns of the wire wound around the core of the magnetic switch SR3.    (6) Voltage VP of the peaking capacitor CP increases as charging proceeds, and when the voltage VP reaches a given value Vb, laser gas between discharging electrodes E is undergone dielectric breakdown, and a main electric discharge is started. This main electric discharge excites a laser medium to generate a laser beam. Before the main electric discharge generates, the laser gas as the laser medium between the electrodes E is pre-ionized by unshown preionization means.    (7) Then, the voltage of the peaking capacitor CP is dropped sharply by the main electric discharge to resume the state before the start of charging.    (8) As the electric discharge operation is repeated by the switching operation of the semiconductor switch SW, pulse laser oscillation is performed at a prescribed repetition frequency.    (9) Here, when it is configured so that the inductance of a capacity migration circuit of each stage configured of the magnetic switch and the capacitor becomes smaller as the stages become near later stages, a pulse compression operation is performed to make the peak value of the current pulse flowing to each stage high sequentially and to make the pulse width sequentially narrow, and an intense electric discharge with a short pulse is realized between the electrodes E. Thus, a glow discharge is stably held between the discharging electrodes, stability of laser emission is enhanced, and an oscillation efficiency of laser is also improved.
In these years, the excimer laser used as an exposure light source is being demanded to perform high repetition discharging at several kHz for increasing a through put. To realize this, it is necessary that the switch SW performs high repetition switching operations. And, it is considered that the reduction of the pulse width by the magnetic pulse compression accelerates start-up of the discharge voltage and enables the high repetition.
FIGS. 14(a) and 14(b) schematically show circuit connection of the magnetic switches (i.e., saturable reactors) SR1 to SR3 and the step-up transformer Tr
The saturable reactors SR1 to SR3 have a winding 2 wound around a magnetic core (hereinafter called the core) 1 which is grounded as shown in FIG. 14(a), and a high voltage is applied to the winding 2. The step-up transformer Tr has a primary winding 3 and a secondary winding 4 wound around the core 1 which is grounded as shown in FIG. 14(b), and when a high voltage is applied to the primary winding 3, a high voltage is generated in the secondary winding 4.
FIG. 15(a) is a perspective diagram showing a state that the winding 2 is wound around the core 1 of the saturable reactor (hereinafter called as the reactor). The core 1 has a magnetic alloy strip 1b wound around a core tube 1a in an annual ring shape. The core shown in the drawing has an annular ring shape but may have the form of a racetrack (oval-shape).
It is necessary to insulate between adjacent turns of the winding 2 and between the winding 2 and the core 1. The reactor to which a high voltage is applied is immersed in insulating oil for insulation and cooling. Therefore, crepe paper 2b having a good oleophilic property is wound as an insulating coating around a core wire 2a as shown in FIG. 15(b). The step-up transformer Tr also has the same configuration excepting that the primary winding and the secondary winding are wound around the core. Therefore, the reactor is mainly described below.
FIG. 16(a) is a diagram conceptually showing a sectional configuration of the above reactor. When a voltage is applied to the winding 2 which is wound around the core 1 having a substantially rectangular cross section as shown in FIG. 16(a), an electric field centers on the edges of the core 1.
This centering of the electric field may cause a corona discharge between the edge and the insulating coating of the winding 2 as shown in FIG. 16(b). When the corona discharge occurs, the insulating coating is damaged gradually, resulting in a short circuit in due course.
In order to prevent the corona discharge, an electric field easing member (hereinafter called as the corona ring) is generally disposed between the edges of the core 1 and the winding 2. FIG. 17 shows a sectional diagram of a fitting configuration of conventional corona rings 5 to the core 1 of the reactor
The corona ring 5 is made of, for example, stainless steel and disposed along all the edges of the four corner of the core 1. Its cross section has an L shape fitting to the edge shape; however, if it has a sharp edge on the surface, an electric field concentrates on it and a corona discharge occurs. Therefore, it is configured to have a smooth curved structure as the whole to ease the electric field.
In FIG. 17, when a voltage is applied to the winding 2, a potential difference is produced in a horizontal direction of top surface A and bottom surface A′ of the core 1. Specifically, the core 1 has a magnetic alloy strip, which has an insulating coating of silica or the like applied to its surface, wound around the core tube in an annual ring shape, and a winding direction of the strip has an electric resistance larger than that of the surface intersecting the winding direction at right angles. Therefore, when a voltage is applied to the winding 2, the potential difference is produced in the winding direction between the top surface A and the bottom surface A′ of the core which are parallel to the winding direction of the strip. Meanwhile, surfaces B on the right and left sides intersecting the winding direction at right angles are held to have substantially the same potential.
Therefore, when conductive corona ring 5 comes into direct contact with the top surface A and the bottom surface A′ of the core 1, a current flows to the corona ring due to the above-described potential difference. Thus, a magnetic flux is cancelled and an effective cross section of the core 1 becomes small
Accordingly, to insulate the core 1 from the corona rings 5, pressboards 6 are placed on the top surface A side and the bottom surface A′ side of the core 1 so to be held between the core 1 and the corona rings 5. The pressboard is formed by pressing multilayered oleophilic paper and generally used as an insulating material in insulating oil. Its thickness is for example 0.75 mm.
In addition, a thick pressboard 7 is placed on each of the corona rings 5 to surround the corona rings 5, and the winding 2 having crepe paper wound therearound is further wound over the pressboards 7.
Generally, a wire-wound apparatus such as a reactor or a step-up transformer generates heat from the core along with the loss of power. A heating value becomes high as the loss becomes large. A temperature increase in the core depends on the number of turns of the winding, a pulse width of a current (voltage) flowing through the winding and a repetition frequency and generally becomes high as these numerical values become larger.
For example, the magnetic switch of the magnetic pulse compression circuit in the discharge pumped laser is used under conditions that the core tends to have a high temperature because, as described above, high repeatability is required and it must be disposed in a small area by, e.g., superposing a plurality of reactors, for downsizing. In such a case, when used at a repetition frequency of 2 kHz for example, the edges of the core being used may have a temperature of 160° C. even when it is being cooled in the insulating oil.
In the configuration of the conventional example shown in FIG. 17, the core 1, the pressboards 6 and the corona rings 5 are closely contacted to each other. Therefore, the cooling medium (insulating oil) in which the reactor is immersed cannot reach between them to fully cool the edges of the core 1. Especially, the core 1 has the magnetic alloy strip, which has an insulating coating such as silica formed on its surface, wound on a core tube in an annual ring shape, so that it has poor thermal conduction in the winding direction of the strip. Therefore, the core portion having the pressboards 6 disposed on the top and bottom surfaces has a temperature higher than the other portions. This temperature increase drastically reduces the service life of the pressboard 6 held between the corona rings 5 and the core 1. The pressboard 6 has a service life of 20 to 30 years at 120° C. but is reported that its service life is halved for every 6.5° C. increase in its temperature.
Therefore, the pressboards 6 have a service life of approximately 3 to 6 months at 160° C. and decompose frequently, resulting in requiring replacement. The corona rings 5 are heated by thermal conduction from the core 1 and the pressboards 7 enclosing the corona rings 5 are also heated. Therefore, a service life of the pressboards 7 disposed on the corona rings 5 also becomes short.
As described above, the conventional wire-wound apparatus such as a reactor or a step-up transformer used for a high-voltage pulse generating circuit had a problem that the pressboard is deteriorated and its service life is shortened by heating.
The present invention was made to solve the above problem of the conventional art, and the object of the present invention is to provide a wire-wound apparatus which has a magnetic core having a magnetic alloy strip wound around a core tube, a winding wound around the magnetic core and used in an insulating cooling medium, wherein the service life of the wire-wound apparatus is increased by configuring to efficiently cool the magnetic core in the vicinity of an electric field easing member disposed at the edges of the magnetic core.